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Number theory Difficulty 3.9 AMC 10/12 Prove it North Macedonia

Find all integers mm for which m3+m2+7m^3 + m^2 + 7 is divisible by m2m+1m^2 - m + 1.

Solution

From m3+m2+7=(m2m+1)(m+2)+(m+5)m^3 + m^2 + 7 = (m^2 - m + 1)(m+2) + (m+5) it follows that m2m+1m^2 - m + 1 is a divisor of m+5m+5. Obviously, one solution is m=5m = -5.

Let m5m \neq -5. Then m2m+1m+5|m^2 - m + 1| \le |m+5|. From m2m+1=(m12)2+34>0m^2 - m + 1 = (m - \frac{1}{2})^2 + \frac{3}{4} > 0 it follows that m2m+1m+5m^2 - m + 1 \le m + 5 or m2m+1m5m^2 - m + 1 \le -m - 5. The case m2m+1m5m^2 - m + 1 \le -m - 5 is not possible (m2+60m^2 + 6 \le 0). From m2m+1m+5m^2 - m + 1 \le m + 5 it follows that m22m40m^2 - 2m - 4 \le 0 i.e. (m1)25(m-1)^2 \le 5. The last inequality is satisfied for m{1,0,1,2,3}m \in \{-1, 0, 1, 2, 3\}. Checking each value, we conclude that m=0m=0 and m=1m=1 satisfy the initial condition.

Finally m{5,0,1}m \in \{-5, 0, 1\}.

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